In the chemical reprocessing of irradiated nuclear fuels, nuclear fuel is dissolved in boiling nitric acid to obtain a nitric acid solution. It is known to place this nitric acid solution in geometrically critically safe annular vessels before extracting useful materials. The fuel solution however still contains solution residues or undissolved chips from the cutting up of fuel elements and corrosion products which, as solids, tend to form a sediment on the bottom of the annular vessel. When emptying annular vessels, deposits of solids have been found which could not be moved by blowing in agitating air.
U.S. Pat. No. 4,844,276 discloses an annular vessel having an annular space for accommodating the suspension which contains the solids. The bottom of the annular vessel is arranged to extend at an angle of inclination. An outlet opening is provided at the lowest point of the vessel bottom and this outlet opening is connected to a discharge line for emptying the vessel. Usually, the liquid is emptied vertically upwardly from the annular vessel so that the bottom and walls can be made without any break-throughs. An annular spray arrangement having nozzle openings is provided in the upper portion of the annular space. After any emptying of the vessel, the solids are flushed away from the vessel wall by means of this spray arrangement and flushed down to the lowest point at the bottom of the vessel.
When the fuel solution placed in intermediate storage in the annular vessel has to be moved to other processing stations, it is desirable that the undissolved solids should, as completely and uniformly as possible, be moved also to a solids-liquids separating device such as a centrifuge and/or a filter. In order to achieve this quantitative movement, it is necessary that the solids be as thoroughly dispersed as possible in the discharged fuel solution in order to produce an even flow of solids to the downstream separating device.
It would be possible to bring the solids into turbulence in the annular vessel by using an agitating air line of annular configuration which is positioned close to the bottom. Air could be blown in through the annular agitator air line and turbulence created to disturb the solids in the liquid. However, using agitating air for this thorough mixing process would entrain considerable quantities of radioactive aerosols into the vessel exhaust system. Furthermore, the compressed air requirement for the agitating air for mixing is very high. Then there is also the disadvantage that with agitating air for mixing, heavy particles may to a certain extent accumulate in specific zones of the flow. This is attributed to the fact that mixing solids with agitating air is inadequate in the case of heavy coarse particles.
An arrangement which prevents downward movement in radioactive fission product solutions in storage tanks is disclosed in German Patent 2,149,425. A dip pipe is connected to a hydraulically or pneumatically operated piston. A gas column is connected between the piston and the dip pipe end so that a pulsating column of liquid is generated. The end of the dip pipe has a widened portion and, on the bottom of the storage tank, a conical seat lies opposite this widened portion. The container described here is a conventional storage tank and not an annular tank. No vacuum discharge or emptying arrangements are provided. In the event of any transfer from this storage tank, the problems already described hereinabove would be encountered.